Pipe molds that are used for centrifugally casting pipe generally comprise an elongated cylindrical member with a "Bell" and "Spigot" end. The "Bell" and "Spigot" are separated by a barrel section.
One of the most commonly used steels for making pipe molds for centrifugally casting pipe is the AISI 4130 grade. This steel grade according to "AISI 4130," Alloy Digest--Data On World Wide Metals And Alloys, Nov. 1954, Revised Mar. 1988, pp. 3, and Kattus, J. R., "Ferrous Alloys--4130," Aerospace Structural Metals Handbook, 1986 Pub., pp. 1-20 can have the chemistries set forth in Table I:
TABLE I ______________________________________ Alloy Digest Aerospace Handbook Element Weight % Weight % ______________________________________ Carbon 0.28-0.33 0.28-0.33 Manganese 0.40-0.60 0.40-0.60 Silicon 0.20-0.35 0.20-0.35 Phosphorous 0.04 max. 0.025 max. Sulphur 0.04 max. 0.025 max. Chromium 0.80-1.10 0.80-1.10 Molybdenum 0.15-0.25 0.15-0.25 Nickel -- 0.25 max. Copper -- 0.35 max. Iron Balance Balance ______________________________________
As is seen by reviewing Table I, conventional pipe mold steels such as the AISI 4130 grade do not contain vanadium.
Conventional thinking has been that pipe mold service life is dependent primarily on the properties of hardness and strength of the as-heat treated pipe mold, therefore, these were the only properties considered for making pipe molds with a long service life.
The element that imparts hardness and strength to pipe mold steels is carbon. Hence, pipe molds intended to have a long service life are made from steels with high carbon level. Consistent with conventional thinking, the AISI 4130 grade had high carbon in the range 0.28-0.33%.
A departure from conventional thinking was to make the carbon level directly related to pipe mold size. Table II is an example this:
TABLE II ______________________________________ Pipe Mold Size Carbon Range Aim ______________________________________ 80 mm (3.2 in.) 0.24-0.29% 0.26% 100 mm (4 in.) 0.24-0.30% 0.27% 150 mm (6 in.) 0.24-0.30% 0.27% 200 mm (8 in.) 0.26-0.31% 0.28% 250 mm (10 in.) 0.27-0.32% 0.29% 350-1200 mm 0.28-0.33% 0.30% (14-40 in.) ______________________________________
The carbon gradient shown in Table II is based on pipe mold size. Small size pipe molds with high carbon have a greater likelihood of either quench cracking during heat treatment or premature failure during service. Larger size pipe molds overcome this by the mass of the pipe molds causing them to cool slower during the quenching step. However, regarding the pipe molds shown in Table II, conventional thinking is followed in that hardness and strength are the primary concerns and high carbon is maintained in the pipe mold steel for that purpose.
There can be problems in fabricating pipe molds from steel that contains high carbon if the carbon is not properly accounted for in the heat treating process. In heat treating pipe molds, the temperature of the pipe mold steel is raised from room temperature to the austenizing temperature, then the pipe mold is water quenched. The microstructure of the pipe mold at this stage is such that the pipe mold is very hard and has a great deal of internal stresses. This quenching step is followed by a tempering step which tempers the hardness, thereby, making the pipe mold softer and alleviating many of the internal stresses. The greater the carbon level in the pipe mold steel chemistry, the greater the hardness and internal stresses. These internal stresses can result in quench cracking during pipe mold manufacture or cracking due to thermal fatigue, and distortion during pipe production.
The present invention is a departure from conventional pipe mold steels as will be explained in detail in the remainder of the specification.